1. Field of the Invention
The present invention relates to a method of pretreating the connecting elements of a gas-tight sleeve-type pipe connection with a tight metal seat and joint shoulders between the pipes to be connected and the sleeve. The pipes are casing or tubing pipes which are made of high-alloy steel and are used in the crude oil and natural gas industries. The method includes coating the sealing and threaded portions of the sleeve with a metal and, prior to screwing the pipes into the sleeve, applying a lubricant on the cleaned sealing and threaded portions of the connecting elements.
2. Description of the Related Art
A pipe connection of a pipeline composed of threaded pipes which can be screwed together and are used in the crude oil and natural gas industries has the purpose, on the one hand, to absorb the load acting on the pipeline and, on the other hand, to ensure the tightness of the connection. For meeting these objects, threaded portions at the ends of the pipes and of the sleeve have, for example, a conical API thread with an appropriate overlap. When the pipes and sleeve are screwed together, this overlap causes high contact pressures. For handling these high loads, surface coating and greases have been developed which make it possible to effect the screw connections without seizing. These thread greases are pastes with lubricating power which have a high content of finely distributed solid particles, such as graphite, metals, or Teflon. In addition to making it easier to screw the pipes and sleeve together, these greases reinforce the hydraulic tightness in the threaded areas of the connection.
In gas-tight sleeve-type pipe connections developed by various manufacturers, the tightness is obtained usually by high contact pressures as a result of an overlap in the tight metal seat.
The joint shoulders have the purpose of serving as stop means for the screw connection and ensure that the tight metal seat remains activated even when the load acting on the pipeline increases. Consequently, the conical thread does not have to perform a sealing function and, contrary to API round and buttress threads, the thread only has the purpose of transmitting the load in the pipeline. In order to be able to reliably screw gas-tight connections together in the threaded and tight-seat areas, in pipe connections in which the pipes are made of unalloyed and middle-alloy carbon steels, an application of phosphate is used almost exclusively as the surface treatment and standardized solid lubrication pastes, such as API grease, are used in order to eliminate seizing phenomena and to make it possible that the screw connections can be used several times.
However, the above-described method cannot be used in high-alloy austenitic steels having nickel and chromium contents of more than 20% by weight and molybdenum content of more than 2.5% by weight.
A known possibility of ensuring that the screw connection can be effected several times in austenitic steels is the implantation of ions in the sealing and threaded areas of the sleeve. In this method, ionized metals, such as palladium, silver, chromium, or gold, penetrate into the surface of the sleeve. After this pretreatment has been carried out, a standardized API grease is applied as a lubricant before the screw connection is made. This expensive pretreatment is only possible in a specialized establishment and poses logistic difficulties for the user and may mean that the user is dependent on a single supplier.